1. Field of the Invention
This invention relates to a hearing aid which amplifies sounds for the hearing impaired.
2. Prior Art
A hearing aid is fundamentally a sound amplifier and usually comprises a microphone, an amplifier and one or two earphones. However, various transducers in place of the earphone have been used recently, such as a bone receiver, a vibrator which contacts the stapes and acicular electrodes which extend into the cochlea and which are applied to middle ear implants or cochlea implants.
The above-mentioned sound amplifier not only amplifies the sound but also processes the sound by using various additional functions.
In detail, a hearing aid processes the sound mainly for frequency and level of the sound elements. For example, a high frequency zone is often emphasized or a loudness level is controlled by a maximum output power control means so that the loudness level does not exceed a loudness threshold which causes discomfort. Thus, the sound is adjusted according to the hearing ability of the hearing impaired.
Usually, the hearing impaired person has a narrower dynamic range compared with that of a person with normal hearing. This dynamic range is defined as the difference obtained when the hearing threshold level (HTL) is subtracted from the threshold of loudness causing discomfort or the uncomfortable loudness threshold (UCL).
UCL is the maximum loudness which the hearer can endure, and HTL is the minimum loudness level which the hearer can recognize.
In the normal case, the UCL is generally between 100 to 110 dB HL (Hearing Level which is measured by an audiometer) and in the case of the hearing impaired, the UCL is generally between 110 and 120 dB HL. Since the dynamic hearing range of a hearing impaired person with mild impairment who has 70 dB HL of HTL is 120 dB-70 dB=50 dB, such a person can understand normal conversation when the speaking voice is amplified by using an ordinary hearing aid so that the voice would be included within the dynamic range.
However, in the case of a hearing impaired person with severe impairment such as 110 dB to 120 dB HL of HTL, the dynamic range will be:
120 dB-100 (or 110) dB=20 dB (or 10 dB).
In such a narrow dynamic range, it is impossible for the person to understand all speech sounds even if the sound level is raised using a hearing aid.
That is, as to vowels, since the difference between the effective value and the peak factor is 15 to 20 dB, the hearing impaired person can hear essentially only the peak factor part, and as to consonants, since the energy level of consonants is lower than that of vowels by approx. 15 to 30 dB, the syllable identification score would be rather low. Further, if the speech sound level is lower than usual, sound information content in excess of the HTL will be decreased and the syllable identification score will be less.
As described above, prior art hearing aids do not perform well enough to compensate for hearing difficulty caused by the narrow dynamic range of the hearing impaired, especially persons with severe impairment, because the prior art hearing aids only amplifys the level of speech sound in various frequency ranges.